Too quick for my taste. We went during transit last month, but we had a 7 hour layover. For short layovers we stay in the secure area. With a US passport, you can go through automated immigration as long as you don't have a child under 6 with you. I've been to Singapore 3 times in the last year and only once have encountered long lines at immigration. Usually no longer than a 5-10 minute wait, haven't experienced many long lines getting back into the secure area either. BUT, depending on where your plane lands and the next one departs, moving between terminals and the jewel can take some time. I wouldn't risk it with a layover that short.